This invention is related generally to temperature and emissivity measurement, and more specifically, to the measurement by pyrometric techniques of high processing temperatures of objects heated by electromagnetic radiation within the visible, infrared or other near-visible wavelength range.
There are many examples of the application of optical heating techniques. One example is in heating materials for the purpose of testing them. Another is in the heat treatment of an object. A further example of is found in the semiconductor processing industry. In this latter example, silicon wafers to be processed are positioned within an enclosed chamber made, at least partially, of an optically transparent material. Lamps outside the chamber direct a large amount of energy through its transparent walls and onto the wafer. The wafer is heated as a result of its absorption of the optical radiation. Generally, the chamber is formed of a quartz envelope, or of stainless steel with an optical window. The heated wafer is treated by introducing appropriate gases into the chamber which react with the heated surface of the wafer.
These processes require that the temperature of the wafer be maintained within narrow limits in order to obtain good processing results. Therefore, some technique of monitoring the temperature of the wafer is required. One possibility is to contact the wafer with a conventional thermocouple, but this is precluded by poor measurement and contamination considerations when semiconductor wafers are the objects being heated. For other types of objects, such contact measurement techniques most often are precluded because of a number of practical considerations. The technique also often results in substantial errors because of a differing thermal mass, poor thermal contact and a difference in emittance between the thermocouple and the object being heated.
As a result, most optical heating applications use some form of a long wavelength pyrometer. This technique measures the intensity of the radiation of the semiconductor wafer or other optically heated object within a narrow wavelength band. That radiation intensity is then correlated with temperature of the object. In order to avoid errors by the pyrometer receiving heating optical radiation reflected from the object being heated, the wavelength chosen for monitoring by the pyrometer is outside of the emission spectrum of heating lamps. This detected wavelength range is generally made to be significantly longer than the spectrum of the lamps.
There are several disadvantages to such existing pyrometric systems. First, a measurement made at a longer wavelength will have only a portion of the sensitivity of one made at a shorter wavelength. Second, the emissivity of silicon and other materials that are optically heated is dependent upon the wavelength at which it is measured. Third, the photodetectors with the highest signal-to-noise ratio are those which respond to the shorter wavelength emissions. Fourth, existing optical pyrometers have a small numerical aperture and thus provide temperature measurements which are also dependent upon the degree of roughness of the object and film growth being measured. Fifth, existing pyrometric techniques are slow, a significant disadvantage in a rapid heating system.
It is a primary object of the present invention to provide an improved pyrometric technique of temperature and/or emissivity measurements that overcomes these shortcomings.